Texture Paint: Pixelated after texture paint
Generally, you should be aware of two things:


The resolution of the background image.
The level of detail of the paint texture.


In your case, I'm not sure how big you build the background image, but if it's 2048 x 2048 px, then, in the current zoom level, the stencil texture may probably covers about 10% of the whole area, which means 648 x 648 px around, far from enough, isn't it? 

Besides, your stencil texture seems sort of a gravel texture, which means the detail unit is quite small. Obviously, you need enough pixels on the background image to at least cover the detail of the a unit gravel, in order to preserve the original detail as you want.

My suggestion is: try not to have the background image over zoomed in while painting.